Known methods for the stabilization of peptides in a solution include a method which comprises allowing a peptide to stand in a solution containing ethylenediaminetetraacetic acid to form an insoluble composition (see patent document No. 1) and a method which comprises allowing plasma or a protein such as γ-globulin or gelatin to be present with a peptide (see non-patent document No. 1).
A known method for the stabilization of freeze-dried peptides is a method which comprises allowing gluconate to be present with a peptide (see patent document No. 2). Saccharides such as sucrose, maltose and lactose are known as stabilizers of freeze-dried peptides (see patent documents Nos. 3 and 4).
Also, in immunoassay, there is known a method for the stabilization of an antigen or antibody immobilized on a solid phase by allowing saccharides such as trehalose and sucrose to be present (see patent documents Nos. 5 and 6).
In general, peptides in a biological sample are unstable due to various factors (e.g., degradation by protease in the biological sample). For the stabilization of peptides in a biological sample, there are known methods which comprise adding a protease inhibitor, a surfactant or an antiseptic. For example, a method in which protease inhibitors (diisopropyl fluorophosphate and phosphoramidon) and bacitracin are allowed to be present is known as a method for the stabilization of substance P in a biological sample (see non-patent document No. 2).
As to the measurement of substance P in a biological sample, known is a method which comprises treating serum with an acid, extracting substance P with acetone and ether, further purifying it by HPLC using a reversed-phase column, and then measuring substance P by enzyme immunoassay (see non-patent document No. 3). However, this method fails to give accurate measurements because the operations are very complicated and chemical treatment conditions are severe.
Peptides in vivo are important as various kinds of markers and their concentrations in biological samples need to be measured accurately. However, there is the problem that in many cases, peptides in collected biological samples have already decreased at the time of measurement, which makes the accurate measurement difficult.
Patent Document No. 1:
    Japanese Published Unexamined Patent Application No. 208485/97Patent Document No. 2:    Japanese Published Unexamined Patent Application No. 321800/94Patent Document No. 3:    Japanese Published Unexamined Patent Application No. 306235/93Patent Document No. 4:    Japanese Published Unexamined Patent Application No. 170664/93Patent Document No. 5:    PCT National Publication No. 508843/02Patent Document No. 6:    Japanese Published Unexamined Patent Application No. 215127/03Non-Patent Document No. 1:    Experientia, Vol. 19, No. 2, p. 72-73, Switzerland (1963)Non-Patent Document No. 2:    Biochemical and Biophysical Research Communications, Vol. 125, No. 2, P. 728-733, U.S.A. (1984)Non-Patent Document No. 3:    Clinical and Diagnostic Laboratory Immunology, Vol. 5, No. 3, p. 303-307, U.S.A. (1998)